Photovoltaic cells and light emitting diodes (LEDs) have a common materials and structural background. Photovoltaic cells receive light from the sun and convert it into usable energy, whereas LEDs convert electrical energy into light. Semiconductors are a key component that enable both these technologies. Semiconducting materials are unique in that their electrical properties can be altered or modified to produce specific outputs. Typically semiconductor devices rely on the movement of charge carriers, such as electrons, within a crystal lattice of atoms. Electrons exist in discrete energy levels, with electrons in the valence energy band being bound to an atom and electrons in the conduction band free to move within the crystal lattice, generating an electric current as a charge carrier. Between these two energy bands is a band gap, an energy level at which no electrons states can exist. The electronic band structure is a characteristic property of the semiconducting material, such as silicon or germanium.
If an electron gains enough energy to overcome the band gap, it can move from the valence band to the conduction band, leaving behind a vacancy in the valence band termed a hole. Electron-hole pairs are the negative and positive charge carriers in semiconductors and are responsible for generating an electric current. Electrons move from the conduction band to the holes in the valence band in a process called recombination. In pure semiconducting material the number of electron-hole pairs are in equilibrium, and the rates of carrier generation and recombination are balanced. In general practice, extrinsic semiconductors are produced when the pure semiconducting material is doped with impurities. Dopant atoms are added that may have surplus electron (n-type) or hole (p-type) concentrations in their valence bands. Thus n-type semiconductors have excess electrons that may move from the valence band to the conduction band, whereas p-type semiconductors have excess holes and can thus accept electrons from the valence band. The accepting or donating of electrons in p-type and n-type materials results in an extrinsic semiconductor having non-neutral charge depending on the dopant material used. The charge equilibrium can thus be selectively modified for use in electronic devices by modifying the rates of carrier generation and recombination. Movement of carriers within extrinsic semiconductors occurs along a potential gradient from the charge differences imparted by n-type and p-type dopants, generating a field current. Currents also arise as carriers move from areas of higher concentration to areas of lower concentration in a diffusion process. Diffusion currents and field currents are not independent, and are often dynamically interacting as charge carrier move through the semiconductor.
The quantum mechanics principles of carrier generation and recombination are important for both LED and photovoltaic cells. In the case of photovoltaic cells, carrier generation can occur through absorption. Photons from sunlight can interact with electrons and excite them to the conduction band, creating a hole in the valence band and generating a free charge carrier. Generally the intensity of the incoming light is proportional to the rate of photon absorption, and not all light that enters a semiconductor will excite an electron to a higher energy band. A similar but reverse process of absorption is termed radiative recombination. Radiative recombination occurs when electrons in the higher energy state of the conduction band recombine with holes in the valence band. During this process, energy is released in the form of a photon—this is the basic operation of an LED.
Each energy band in a semiconductor is characterized by a momentum vector related to the crystal structure of the semiconductor material. For some crystal lattice structures, the maximum energy state of the valence band and the minimum energy state of the conducting band have the same crystal momentum vector, and are termed direct band gap semiconductors, whereas if the crystal momentum vector is different they are indirect band gap semiconductors. The direct or indirect band gap nature of semiconductors has implications for photon absorption and radiative recombination. In indirect band gap materials, such as silicon, an electron needs a change in momentum in addition to a change in energy for light absorption or radiative recombination. Photons transmit energy to electrons but nearly zero momentum, so there is a much lower probability of absorption and radiative recombination in indirect band gap materials. This typically makes indirect band gap materials a poor choice for LEDs, as the effects of radiative recombination (emission of a photon) forms the basis for their operation. The common semiconductor material silicone is an indirect band gap material, and although it is much more common and easily produced than direct gap materials, it is highly inefficient in terms of absorption and radiative recombination and not a typical choice for creating LEDs. Many of the high costs of solar cell production comes from the thickness of the materials required to build them. In addition to size alone, the increased thickness makes solar cells heavy and rigid, limiting where and how they can be deployed and implemented with other technology, such as handheld devices or similar applications. There exists a need for a silicon based, high-voltage thin solar cell and light emitter.